Method of fabricating a traveling wave tube

ABSTRACT

A hollow cylindrical slow wave circuit, such as a helix, is glued to its associated support structure, such as three longitudinally directed dielectric rods spaced at 120* intervals about its periphery. In addition, the slow wave circuit is glued to a mandrel inserted within the slow wave circuit and extending axially thereof. The mandrel is relieved in the peripheral regions juxtapositioned the support rods to allow inward deflection of the slow wave circuit and support structure. The glued sub assembly is then axially inserted within an elongated bore in the body portion of a traveling wave tube. The bore and the sub assembly are sized relative to each other to obtain a slidable interference fit. The relief in the mandrel permits slight inward deflections of the slow wave circuit such that the slow wave circuit and support structure are captured by means of the interference fit within the bore of the body of the tube.

United States Patent n91 Eallonardo METHOD OF FABRICATING A TRAVELING WAVE TUBE [75] Inventor: Charles M. Eallonardo, Palos Verdes Peninsula, Calif.

[73] Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: Oct. 10, 1972 [21] Appl. No.: 296,494

Primary Examiner-Charles W. Lanham Assistant Examiner-Robert M. Rogers Attorney, Agent, or Firm-Stanley Z. Cole; Harry E. l-line; Robert K. Stoddard [4 1 May 7,1974

[57] ABSTRACT A hollow cylindrical slow wave circuit, such as a helix, is glued to its associated support structure, such as three longitudinally directed dielectric rods spaced at 120 intervals about its periphery. In addition, the slow wave circuit is glued to a mandrel inserted within the slow wave circuit and extending axially thereof. The mandrel is relieved in the peripheral regions juxtapositioned the support rods to allow inward deflection of the slow wave circuit and support structure. The glued sub assembly is then axially inserted within an elongated bore in the body portion of a traveling wave tube. The bore and the sub assembly are sized relative to each other to obtain a slidable interference fit. The relief in the mandrel permits slight inward deflections of the slow wave circuit such that the slow wave circuit and support structure are captured by means of the interference fit within the bore of the body of the tube.

6 Claims, 4 Drawing Figures PATENTEO 7 I974 FIG.|

ASSEMBLE HELIX A v AND RODS ON TRIANCULAR MANDREL AND CLAMP IN PLACE r APPLY CLUE AND LET DRY TO FORM SUBASSEMBLY LAP AND GRIND SUBASSEMBLY FOR INTERFERENCE I INSERT SUBASSEMBLY INTO TUBE BODY I DISSOLVE CLUE AND REMOVE MANDREL COMPLETE TU ASSEMBLY AND PROCESS- TUBE METHOD OF FABRICATING A TRAVELING WAVE TUBE DESCRIPTION OF THE PRIOR ART Heretofore', slow wave circuits and their associated dielectric support rods have been captured by means of an interference fit within the bore of the body of a traveling wave tube. Such a method of fabricating a traveling wave tube is disclosed and claimedin U.S. Pat. No. 3,540,119 issued Nov. 17, l970-and assigned to the same assignee as the present invention.

In this prior art method, the helix was glued to three longitudinally directed dielectric support rods spaced at approximately 120 intervals around the periphery of the helix. The helix was also glued to a cylindrical mandrel coaxially disposed within the hollow interior of the helix. The mandrel was hollow and had fluid cooling channels therein to allow cooling of the mandrel in use. The body of the tube was heated to approximately 800 to expand the inside diameter of the bore which was to receive the helix and support structure. The glued sub assembly, as glued to the mandrel, was then axially-inserted within the longitudinal bore in the body of the tube. Due to the cooling of the mandrel and sub assembly and due to the relatively high temperature of the body of the tube, a substantial clearance was obtained between the sub assembly and the bore. After the sub assembly was inserted, the body of the tube was allowed to cool thereby shrinking the inside diameter of the bore and producing an interference fit between the sub assembly and the inside wall of the bore in the body of the tube. The assembly was then washed with a solvent for the glue to dissolve the glue such that the mandrel could be removed leaving the slow wave circuit and support structure captured by an interference fit within the body of the tube. The tube was then assembled andprocessed in the conventional manner.

The problem with the prior art method for insertion of a slow wave circuit in a traveling wave tube is that when the dimensions of the circuit are reduced to those dimensions required for X-band, the inside diameter of the bore in the body of the tube is reduced to the dimension of approximately 0.070 inches and the helix will have an outside diameter of approximately 0.040

inches. Heating the body of the tube to 800 C does not appreciably increase the inside diameter of a bore having a diameter of only 0.070 inches. Also the diameter of the mandrel is only approximately 0.035 inches and it becomes exceedingly difficult to provide sufficient cooling for the mandrel and associated helix and support structure.

Therefore it is desirable to provide an improved method for inserting a slow wave circuit into the body of a microwave traveling wave tube.

SUMMARY OF THE PRESENT INVENTION to a centrally disposed mandrel which is relieved in perpheral regions thereof disposed juxtaposition the support structure, thereby forming a sub assembly which is inserted by means of a slidable interference fit into the bore of the body of the tube.

In another feature of the present invention, the glued sub assembly is sized, as by lapping and grinding, to obtain an interference fit of approximately 0.0004 inches between the outside diameter of the sub assembly and the inside diameter of the bore in the body of the tube which is to receive the sub assembly.

In another feature of the present invention, the slow wave circuit is a helix, the support structure for supporting the slow wave circuit within the body of the tube comprises three longitudinally directed rods spaced at l20 intervals around the circumference of the helix, and the mandrel is of generally triangular cross-section with the apex portions of the mandrel being glued to the slow wave circuit circumferentially intermediate adjacent ones of said circumferentially spaced support rods.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings.

' BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow diagram, in block diagram form, depicting the method for fabricating a slow wave tube according to the present invention,

FIG. 2 is an enlarged transverse sectional view of the slow wave circuit, dielectric support rods, and mandrel as jigged for gluing,

FIG. 3 is a reduced view of a portion of the structure of FIG. 2 taken along line 3-3 in the direction of the arrows, and

FIG. 4 is an enlarged cross-sectional view of the glued sub assembly including the slow wave circuit, dielectric support rods, and mandrel as inserted within the bore in the body portion of the tube.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown, in block diagram form, the method of the present invention. More particularly, in step (a) and as shown in FIGS. 2 and 3, a hollow cylindrical slow wave circuit 1, such as a helix, ring and bar, bifilar helices, cross-wound helices or the like, is assembled with its dielectric support rod 2, as of boron nitride, in a jig fixture having three sets of split comb-shaped clamps 3. The upper clamp 3' clamps the upper rod 2 and is vertically adjustable to permit insertion of the lower rods 2 and helix 1 before the upper clamp is lowered into the position as shown in FIG. 2. The clamps 3 hold the dielectric support rods at intervals about the periphery of the hollow cylindrical slow wave circuit 1.

A mandrel 4 is inserted coaxially of the slow wave circuit 1. The mandrel is relieved about its periphery at regions 5 which are peripherally juxtapositioned the three dielectric support rods 2. Thus, in cross-section the mandrel 4 is generally triangular with the apexes of the triangle disposed circumferentially intermediate the rods 2 and such apexes being rounded to conform to the curvature of the inside wall of the hollow slow wave circuit 1.

In a typical example, the rods 2 have a length as of 2 to 5 inches and are of square cross-section 0.014

inches on a side.

The helical slow wave circuit 1 is made of a tungsten tape 0.003 inches thick and 0.007 inches wide. The inside diameter of the helix is 0.039 inches, there being approximately95 turns per linear inch of the helix. The rods 2 may be coated with an r.f. lossy material in certain regions for attenuating undesired backward wave energy in the conventional manner.

In step (b), glue is applied by means of a paint brush, with the aid of a microscope, to the adjoining corners of the side edges of the dielectric support rods 2 and the helix at glue lines as indicated at 6. The longitudinally spaced clamps 3 permit access to the periphery of the sub assembly for brushing on the glue. Thus the rods 2 are glued to the outside periphery of the slow wave circuit 1. In addition, the slow wave circuit 1 is glued to the mandrel 4 by brushing glue onto the abutting portions of the helical tape 1 and the rounded apex portions 7 of the mandrel 4 along glue lines at 8. A suitable glue is selected from the class consisting of methacrylate lacquer and butylmethacylatc. The glue is allowed to dry and the glued sub assembly is removed from the clamps 3.

In step (c), the outside diameter of the glued sub assembly is lapped and ground to provide an interference fit of approximately 0.0004 inches with the honed inside wall of a longitudinal borell (see FIG. 4) in the metallic body 12 of the traveling wave tube.

In step (d), the lapped and ground sub assembly is axially inserted within the bore 11 in the body of the tube 12. Due to the interference fit, the rods 2 will produce a slight inward deflection of the slow wave circuit 1, such deflection being accommodated by the relieved portions of the mandrel 4. The amount of deflection is well within the elastic limit of a tungsten slow wave circuit 1 and serves to assure a tight interference fit of the sub assembly within the bore 11.

In step (e), the body 12 containing the glued sub assembly is washed with a solvent for the glue such as acetone to remove the glue. After the glue has been removed the mandrel 4 may be removed from the slow wave circuit 1.

In step (f), the tube body portion 12 containing the helix 1 and support rods 2 is assembled with remaining portion of the tube and the tube is processed in the conventional manner.

The advantage to the method of the present invention is that it allows assembly of the slow wave circuit and its supporting structure within the bore of a traveling wave tube without having to heat the body of the tube or to cool the mandrel and yet achieves a tight thermally conductive interference bond between the supporting structure and the inside wall of the bore 11, while also providing a tight interference fit between the support structure 2 and the slow wave circuit 1. These tight interference joints greatly facilitate the transfer of heat from the slow wave circuit to the body of the tube,

in use.-

Using this method of construction, X-band traveling wave tubes have been fabricated to yield watts cw power output. The slight distortion of the helix does not deleteriously affect the performance of the tube. Although the circuit suppoxt structure 2 has, thus far, been described as dielectric rods, this is not a requirement as the support structure may take other shapes of dielectric material and in certain cases may be metallic.

What is claimed is:

1. In a method for fabricating a traveling wave tube the steps of:

gluing a hollow generally cylindrical slow wave circuit to a plurality of axially directed support structures disposed at circumferentially spaced positions about the periphery of said slow wave circuit and, to a centrally disposed axially directed mandrel which is spaced away from said slow wave circuit in those circumferentially spaced, axially extending regions which face said support structures, thereby forming a glued sub assembly; axially inserting the glued sub assembly into an elongated bore in the body portion of a traveling wave tube, said bore and sub assembly being sized relative to each other to obtain a slidable interference fit between said sub assembly and the inside wall of said bore;

removing the glue; and

removing the mandrel to leave the slow wave circuit and said associated support structure captured by means of said interference fit within said bore in said body portion of said tube.

2. The method of claim 1 wherein the step of gluing the support structure to the slow wave circuit includes, gluing three dielectric support rods to the periphery of the slow wave circuit proximate 120 intervals about the circumference of the slow wave circuit; and wherein the mandrel is generally triangular in crosssection with the apex portions of said mandrel being rounded to conform to the curvature of the inside wall of said slow wave circuit; and wherein the step of gluing the mandrel to the slow wave circuit includes the step of, gluing the rounded apex portions of said mandrel to said slow wave circuit at points disposed generally circumferentially midway between adjacent ones of said circumferentially spaced rods.

3. The method of claim 2 wherein the slow wave circuit is a helix.

4. The method of claim 1 wherein the step of removing the glue includes the step of, washing the glued sub assembly mounted in the bore in the body of the tube with a solvent for the glue.

5. The method of claim 1, wherein the glue is selected from the class consisting of methacrylate lacquer and butylmethacrylate.

6. The method of claim 1 including the additional step of, sizing the slow wave circuit sub assembly after the step of gluing and before the step of inserting the sub assembly into the bore of the body portion of the microwave tube. 

1. In a method for fabricating a traveling wave tube the steps of: gluing a hollow generally cylindrical slow wave circuit to a plurality of axially directed support structures disposed at circumferentially spaced positions about the periphery of said slow wave circuit and, to a centrally disposed axially directed mandrel which is spaced away from said slow wave circuit in those circumferentially spaced, axially extending regions which face said support structures, thereby forming a glued sub assembly; axially inserting the glued sub assembly into an elongated bore in the body portion of a Traveling wave tube, said bore and sub assembly being sized relative to each other to obtain a slidable interference fit between said sub assembly and the inside wall of said bore; removing the glue; and removing the mandrel to leave the slow wave circuit and said associated support structure captured by means of said interference fit within said bore in said body portion of said tube.
 2. The method of claim 1 wherein the step of gluing the support structure to the slow wave circuit includes, gluing three dielectric support rods to the periphery of the slow wave circuit proximate 120* intervals about the circumference of the slow wave circuit; and wherein the mandrel is generally triangular in cross-section with the apex portions of said mandrel being rounded to conform to the curvature of the inside wall of said slow wave circuit; and wherein the step of gluing the mandrel to the slow wave circuit includes the step of, gluing the rounded apex portions of said mandrel to said slow wave circuit at points disposed generally circumferentially midway between adjacent ones of said circumferentially spaced rods.
 3. The method of claim 2 wherein the slow wave circuit is a helix.
 4. The method of claim 1 wherein the step of removing the glue includes the step of, washing the glued sub assembly mounted in the bore in the body of the tube with a solvent for the glue.
 5. The method of claim 1, wherein the glue is selected from the class consisting of methacrylate lacquer and butylmethacrylate.
 6. The method of claim 1 including the additional step of, sizing the slow wave circuit sub assembly after the step of gluing and before the step of inserting the sub assembly into the bore of the body portion of the microwave tube. 